1. Field of the Invention
The present invention relates to a high-frequency circuit for controlling attenuation of an attenuating circuit including a PIN diode in accordance with an input signal level.
2. Description of the Related Art
In existing broadcast signal reception systems, an input signal, such as a television signal, received by an antenna is input to an attenuating circuit while the attenuation of the attenuating circuit is controlled by an AGC voltage provided by a detection circuit. When the input level of the television signal is too high, control is performed so that the attenuation increases. When the input level of the television signal is too low, control is performed so that the attenuation decreases. Thus, the input level of the television signal can be maintained to be a predetermined level. The input signal output from the attenuating circuit is amplified by a low-noise amplifier and is input to a downstream frequency conversion circuit.
FIG. 7 illustrates an example of a high-frequency circuit including an attenuating circuit disposed upstream of a low-noise amplifier. A television signal received by an antenna is delivered to an input terminal 10a of a television signal reception integrated circuit. One end of a signal line 11 is connected to the input terminal 10a whereas the other end of the signal line 11 is connected to an input end of a low-noise amplifier 12. A PIN diode D2 is disposed in the signal line 11. An anode of the PIN diode D2 is connected to the input end of the low-noise amplifier 12. A PIN diode D1 is connected between part of the signal line 11 on the cathode side of the PIN diode D2 and ground. A fixed voltage is generated by a direct current bias circuit composed of resistors R1 and R2. The fixed voltage is applied to an anode of the PIN diode D1 via a resistor 13. Another fixed voltage is generated by a bias circuit composed of resistors R3, R4, and R5. This fixed voltage is applied to the input end of the low-noise amplifier 12. Additionally, a bias circuit composed of a resistor R6 and a coil 14 applies an AGC voltage to the anode of the PIN diode D2 so as to drive the PIN diode D2 using the AGC voltage.
An antenna tuning circuit 21, an RF amplifier 22, an RF tuning circuit 23, a mixer 24, an IF tuning circuit 25, and an IF amplifier 26 that form a tuner unit are provided at an output end of the low-noise amplifier 12. An output end of the IF amplifier 26 is connected to an output terminal 10b of the television signal reception integrated circuit. A VIF integrated circuit 28 is connected to the output terminal 10b via an SAW filter 27. The VIF integrated circuit 28 incorporates an AGC circuit that generates an AGC voltage so as to output the AGC voltage in accordance with an input electric field. The AGC voltage is input to the bias circuit, which is dedicated to the PIN diode D2, and the RF amplifier 22. FIG. 8A is a characteristic diagram illustrating a relationship between the AGC voltage of the RF amplifier 22 and the attenuation. In the RF amplifier 22, the attenuation is very small in a low-level electric field range in which the AGC voltage is higher than about 3.5 V. However, the attenuation is gradually increased in a range from a medium electric field to a high electric field in which the AGC voltage is lower than 3.5 V.
The PIN diode D2 is a current-controlled diode. Therefore, in order to drive the PIN diode D2 using a direct current, an electric current of several mA needs to be applied to an AGC voltage supply line L1. The current capacity of the AGC circuit in the VIF integrated circuit 28 is smaller than that required for driving the PIN diode D2. Accordingly, the current capacity is increased by providing an AGC buffer amplifier 29 or 29′ inside or outside the television signal reception integrated circuit.
When the AGC voltage is at a high level, a high voltage is applied from the bias circuit (the resistor R6 and the coil 14) to the anode of the PIN diode D2. Therefore, the PIN diode D2 becomes electrically conductive so that a large amount of electric current flows in the PIN diode D2. The input signal is input to the low-noise amplifier 12 without a decrease in the level thereof and is amplified. Furthermore, the input signal is amplified by the RF amplifier 22 to which the same AGC voltage is applied with high gain. In contrast, when the AGC voltage is at a low level, a low voltage is applied from the bias circuit (the resistor R6 and the coil 14) to the anode of the PIN diode D2. Therefore, a small amount of electric current flows in the PIN diode D2. Thus, an amount of electric current flowing to ground via the PIN diode D1 is relatively increased. As a result, the signal level input to the low-noise amplifier 12 is decreased. In this way, the attenuation is controlled in conjunction with the AGC voltage.
A high-frequency circuit is proposed that includes an attenuating circuit having a PIN diode upstream of an amplifying circuit so as to change the attenuation of the attenuating circuit in accordance with the input signal level (refer to, for example, Japanese Unexamined Patent Application Publication No. 10-065469)
However, in the above-described existing high-frequency circuit, the low-noise amplifier 12 cannot respond to a change in the input signal level because the bias voltage of the low-noise amplifier 12 is fixed, and therefore, the sensitivity of reception may deteriorate. In order for the low-noise amplifier 12 to respond a change in the input signal level, the low-noise amplifier 12 requires an additional circuit for supplying an AGC voltage to the low-noise amplifier 12. Therefore, the circuit scale may be disadvantageously increased.
In addition, to drive the PIN diode D2 disposed in the signal line 11, the existing high-frequency circuit requires the AGC buffer amplifier 29 or 29′ inside or outside the television signal reception integrated circuit. Accordingly, the circuit scale is disadvantageously increased. In addition, standardization of circuits is disadvantageously prevented.
Furthermore, the operating point of the PIN diode D2 disposed in the signal line 11 cannot be freely set because the PIN diode D2 is directly driven by the AGC voltage. FIG. 8B illustrates the attenuation characteristic of an attenuating circuit having the PIN diode D2. As can be seen from FIGS. 8A and 8B, when the AGC voltage is greater than about 3.4 V, the attenuation is very small. However, when the AGC voltage is less than about 3.4 V, the attenuation starts to increase in the range from the low electric field to the medium electric field. It is desirable that the input signal is not attenuated in the range from the low electric field to the medium electric field, and the input signal is attenuated by operating the attenuating circuit from the high electric field range in which, for example, the AGC voltage is less than 1.5 V. However, even when the attenuation starts in the range from the low electric field to the medium electric field, the operating point cannot be freely determined. Therefore, the sensitivity of reception may deteriorate when a medium electric field is input.